Fix silent data loss in function_call arguments/output non-string JSON parsing

responsesInputToMessages unmarshaled function_call.arguments and
function_call_output.output as bare strings, silently dropping the
value when it arrived as an object or content-parts array. This
caused the model to lose tool-call context in multi-turn
conversations, increasing the likelihood of malformed tool-call JSON.

Add rawJSONToString (re-encodes non-string values as JSON strings)
and outputToString (extracts text from content-parts arrays, re-encodes
other non-string values). Add 3 regression tests covering object
arguments, array output, and object output.

internal/openai/responses.go

@@ -3,6 +3,7 @@ package openai
 import (
 	"encoding/json"
 	"errors"
+	"strings"
 )
 
 // --- Responses API request parsing & conversion ---
@@ -194,9 +195,7 @@ func responsesInputToMessages(input json.RawMessage) ([]map[string]any, error) {
 				inner["name"] = name
 			}
 			if v, ok := item["arguments"]; ok {
-				var args string
-				_ = json.Unmarshal(v, &args)
-				inner["arguments"] = args
+				inner["arguments"] = rawJSONToString(v)
 			}
 			if v, ok := item["call_id"]; ok {
 				var id string
@@ -215,9 +214,7 @@ func responsesInputToMessages(input json.RawMessage) ([]map[string]any, error) {
 				msg["tool_call_id"] = id
 			}
 			if v, ok := item["output"]; ok {
-				var out string
-				_ = json.Unmarshal(v, &out)
-				msg["content"] = out
+				msg["content"] = outputToString(v)
 			}
 			messages = append(messages, msg)
 		default:
@@ -279,6 +276,58 @@ func convertContentParts(raw json.RawMessage) any {
 	return result
 }
 
+// rawJSONToString converts a json.RawMessage to a string. If the value is
+// already a JSON string, it is used directly. Any other JSON value (object,
+// array, number, bool) is re-encoded as a JSON string so it can populate
+// fields that require a string, such as tool_calls[].function.arguments.
+// This prevents silent data loss when a field arrives as a non-string type.
+func rawJSONToString(v json.RawMessage) string {
+	var s string
+	if json.Unmarshal(v, &s) == nil {
+		return s
+	}
+	var anyValue any
+	if json.Unmarshal(v, &anyValue) == nil {
+		if b, err := json.Marshal(anyValue); err == nil {
+			return string(b)
+		}
+	}
+	return string(v) // last resort: raw bytes
+}
+
+// outputToString converts a function_call_output "output" value to a string
+// for the Chat Completions tool message content. The output may be:
+//   - a plain string (used directly)
+//   - an array of content parts (text extracted and concatenated)
+//   - any other JSON value (re-encoded as a JSON string)
+func outputToString(v json.RawMessage) string {
+	var s string
+	if json.Unmarshal(v, &s) == nil {
+		return s
+	}
+	// Try array of content parts — extract text from each part.
+	var parts []map[string]any
+	if json.Unmarshal(v, &parts) == nil {
+		var sb strings.Builder
+		for _, p := range parts {
+			if t, _ := p["text"].(string); t != "" {
+				sb.WriteString(t)
+			}
+		}
+		if sb.Len() > 0 {
+			return sb.String()
+		}
+	}
+	// Fallback: re-encode as a JSON string.
+	var anyValue any
+	if json.Unmarshal(v, &anyValue) == nil {
+		if b, err := json.Marshal(anyValue); err == nil {
+			return string(b)
+		}
+	}
+	return string(v)
+}
+
 // translateResponsesTools converts tools from the Responses API flat format
 // to the Chat Completions nested {type:"function",function:{…}} format.
 func translateResponsesTools(raw json.RawMessage) (json.RawMessage, error) {